We construct the first globally consistent semirealistic type I string vacua on an elliptically fibered manifold where the zero modes of the Euclidean $D1$-instanton sector allow for the generation of nonperturbative Majorana masses of an intermediate scale. In another class of global models, a $D1$-brane instanton can generate a Polonyi-type superpotential breaking supersymmetry at an exponentially suppressed scale.

• Received 5 December 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.251601